Maternal embryonic leucine zipper kinase: A novel biomarker and a potential therapeutic target in lung adenocarcinoma

Abstract

Maternal embryonic leucine zipper kinase (MELK), is an adenosine monophosphate-activated protein kinase-related kinase that serves important roles in tumourigenesis in multiple malignant tumours. However, to the best of our knowledge, the effect of MELK in lung adenocarcinoma (LUAD) has not been elucidated. The present study aimed to explore the clinical significance of MELK in the prognosis of LUAD. Data from Oncomine, Gene Expression Profiling Interactive Analysis (GEPIA) and The Cancer Genome Atlas (TCGA) were selected to predict the differential mRNA expression levels of MELK mRNA in LUAD and normal tissues. Subsequently, LUAD and adjacent normal tissue samples were collected from 75 patients with the disease, and immunohistochemistry was used to detect the protein expression of MELK. In addition, the Kaplan-Meier Plotter database, GEPIA and TCGA were used to verify the effect of MELK expression on clinical prognosis in patients with LUAD. MELK was significantly upregulated in LUAD tissues compared with that in normal tissues based on Oncomine, GEPIA and TCGA data (P<0.05). In addition, the results from immunohistochemistry demonstrated that the MELK protein level in LUAD tissues was significantly higher compared with that in matched normal tissues (P<0.05). Prognostic analysis performed using the Kaplan-Meier plotter, GEPIA and TCGA suggested that the expression of MELK was negatively associated with the overall survival time of patients with LUAD (P<0.05). In conclusion, MELK was highly expressed in LUAD based on bioinformatics and immunohistochemistry analysis, and increased expression of MELK was associated with a poor patient prognosis. MELK may serve as a potential diagnostic marker and therapeutic target for LUAD.

Keywords: MELK; bioinformatics; immuno-histochemistry; lung adenocarcinoma; prognosis.

Figure 1.

MELK expression in different cancer types was analysed with Oncomine. The expression of MELK in different cancers is summarized in the figure. The numbers of unique comparisons satisfying the thresholds (P<0.01; fold change ≥2; gene rank ≤10%; and data type, mRNA) are displayed in the coloured cells. Red denotes upregulation of MELK in tumour tissues compared with normal tissues, whereas blue indicates decreased expression of MELK in tumour tissues compared with normal tissues. The gene rank is depicted by the colour gradient in the cells. CNS, central nervous system; MELK, maternal embryonic leucine zipper kinase.

Figure 2.

Levels of MELK expression in LUAD based on data from the Oncomine database. (A) Meta-analysis of the 7 analyses on MELK mRNA levels in LUAD identified via Oncomine. (B) mRNA expression of MELK in LUAD using Oncomine database. (a) MELK mRNA expression in LUAD in Hou Lung dataset. (b) MELK mRNA expression in LUAD in Landi Lung dataset. (c) MELK mRNA expression in LUAD in Okayama Lung dataset. (d) MELK mRNA expression in LUAD in Beer Lung dataset. (e) MELK mRNA expression in LUAD in Selamat Lung dataset. (f) MELK mRNA expression in LUAD in Stearman Lung dataset. (g) MELK mRNA expression in LUAD in Su Lung dataset. The X axis of the plot represents the normal vs. cancer group, and the Y axis represents log2-transformed, median/mean centred mRNA expression. The line in the middle represents the median value. LUAD, lung adenocarcinoma; MELK, maternal embryonic leucine zipper kinase.

Figure 3.

Levels of MELK mRNA expression in LUAD based on TCGA and GEPIA databases. (A) MELK expression in LUAD in TCGA data. The X axis of the plot represents the normal vs. cancer group and the Y axis represents mRNA expression in transcripts per million. (B) MELK expression in patients with LUAD based on gender in TCGA data. The x-axis of the plot represents the normal vs. cancer group and the y-axis represents mRNA expression in transcripts per million. (C) MELK expression in different age groups of LUAD in TCGA data. The x-axis of the plot represents the normal vs. cancer group, and the y-axis represents mRNA expression in transcripts per million. (D) MELK expression in different N stages of LUAD in TCGA data. The x-axis of the plot represents the normal vs. cancer group and the y-axis represents mRNA expression in transcripts per million. (E) MELK expression in LUAD in GEPIA data. The x-axis of the plot represents the normal vs. cancer group and the y-axis represents mRNA expression in transcripts per million. (F) MELK expression in different stages of LUAD in the GEPIA database. The x-axis of the plot represents different stages of cancer and the y-axis shows mRNA expression in transcripts per million. Unpaired student's t-test or ANOVA with a post hoc Tukey's honest significant difference test was used to compare the differential expression of MELK mRNA in different groups as appropriate. *P<0.05. N, node; LUAD, lung adenocarcinoma; MELK, maternal embryonic leucine zipper kinase; TCGA, The Cancer Genome Atlas; GEPIA, Gene Expression Profiling Interactive Analysis.

Figure 4.

Changes in the expression of MELK protein in tumour and normal tissues from patients with LUAD. (A) Haematoxylin and eosin staining of LUAD tissues (original magnifications, ×50 and ×200). (B) MELK expression in LUAD was detected by immunohistochemistry (original magnifications, ×50 and ×200). (C) Quantification of positive staining for MELK in LUAD and normal tissues. (D) High expression of MELK in LUAD (n=75). A Student's t-test was used to compare the expression of MELK between LUAD and normal tissues. *P<0.05 vs. non-cancer. LUAD, lung adenocarcinoma; MELK, maternal embryonic leucine zipper kinase.

Figure 5.

OS rate of patients with LUAD with low and high MELK mRNA expression according to Kaplan-Meier plotter, GEPIA and TCGA analyses. (A) GEPIA analysis for OS in patients with LUAD with low and high levels of MELK expression. (B) Kaplan-Meier plots for OS in patients with LUAD according to MELK expression. (C and D) TCGA analysis for OS in patients with low and high levels of MELK expression. Samples with MELK expression values above the median value were categorized into the high MELK group and those with MELK expression values below the median value were categorized into the low MELK group. The red line represents patients with high MELK expression and the blue and black lines represent patients with low MELK expression. The x-axis represents the number of patients at risk at a specific time (in months or days) and the y-axis represents OS time (Kaplan-Meier plotter and GEPIA for moths, TCGA for days). Log-rank P-values and HR with 95% CI were calculated and displayed on the webpage. LUAD, lung adenocarcinoma; MELK, maternal embryonic leucine zipper kinase; TCGA, The Cancer Genome Atlas; GEPIA, Gene Expression Profiling Interactive Analysis; CI, confidence interval; HR, hazard ratio; OS, overall survival.